1. Technical Field
This disclosure relates to an image forming apparatus, and more specifically to an image forming apparatus including a recording head for ejecting liquid droplets.
2. Description of the Background Art
Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or multi-functional devices having two or more of the foregoing capabilities. As one type of image forming apparatus employing a liquid-ejection recording method, an inkjet recording apparatus is known that uses a recording head (liquid-droplet ejection head) for ejecting droplets of ink. During image formation, the inkjet recording apparatus ejects droplets of ink or other liquid from the recording head onto a recording medium to form a desired image.
Such liquid-ejection-type image forming apparatuses fall into two main types: a serial-type image forming apparatus that forms an image by ejecting droplets from the recording head while moving the carriage with the recording head in a main scanning direction, and a line-head-type image forming apparatus that forms an image by ejecting droplets from a linear-shaped recording head held stationary in the image forming apparatus as the recording medium is conveyed thereto.
As for the recording heads used in these liquid-ejection-type image forming apparatuses, several different types are known. One example is a piezoelectric recording head that ejects droplets by deforming a diaphragm using, e.g., a piezoelectric actuator. When the piezoelectric actuator deforms the diaphragm, the volume of a chamber located behind the diaphragm and containing the liquid is changed. As a result, the internal pressure of the chamber increases, thus ejecting droplets from the head. Another example is a thermal recording head that ejects droplets by increasing the internal pressure of the chamber using a heater. This increase is accomplished, for example, by using a heater located in the chamber that is heated by an electric current to generate bubbles in the chamber. As a result, the internal pressure of the chamber increases, thus ejecting droplets from the head.
For such liquid-ejection type image forming apparatuses, there is demand for enhancing throughput, i.e., speed of image formation. One way to achieve enhanced throughput is to enhance the efficiency of liquid supply. For example, a tube supply method is proposed in which ink is supplied from a high-capacity ink cartridge (main tank) mounted in the image forming apparatus to a head tank (also referred to as a sub tank or buffer tank) mounted in an upper portion of the recording head through a tube.
In the tube supply method, because ink ejected from the recording head during image formation is supplied from the ink cartridge to the recording head through the tube, for example, use of a flexible narrow tube increases fluid resistance against ink passing through the tube. As a result, ink may not be timely supplied in an adequate amount to the recording head, thus causing ejection failure. In particular, in a case in which a large-size image forming apparatus that records images onto recording media having large widths employs the tube supply method, a relatively long tube is required, thus further increasing the resistance of the tube against ink flow. Moreover, high-speed printing and/or ejection of highly viscous ink may increase the resistance of the tube against ink flow, thus causing shortage of ink supplied to the recording head.
To counteract these problems, a conventional liquid ejection apparatus is proposed that has a differential pressure valve (negative-pressure conjunction valve) disposed upstream from the recording head in the ink supply direction. The liquid ejection apparatus maintains ink in the ink cartridge in a pressurized state, and supplies ink with the differential pressure valve when the negative pressure within the sub tank exceeds a threshold value.
Further, other techniques for dealing with pressure loss due to fluid resistance of the tube include actively controlling the ink supply pressure by using a pump with or without a negative-pressure chamber that maintains an internal negative pressure with a spring disposed upstream from the recording head in the ink supply direction.
Although generally effective for resolving the above-described problem of poor ink replenishment, the controls and mechanisms employed for controlling the pumps that supply the ink and the negative pressure are complex and the negative-pressure conjunction valve needs a superior seal, as with all connecting portions in the ink supply tubes. A reduction in the sealing performance may result in ink leakage from the joints. Further, because the sending amount of ink of the pump need be controlled in accordance with the consumption amount of ink and other factors, complex control, such as feedback control utilizing the pressure in the negative-pressure chamber, may be needed.
Alternatively, in a case in which the above-described conventional techniques are applied to an image forming apparatus that forms images with several different colors of ink, the pump need be separately controlled for each color, thus resulting in complex configuration and upsizing of the apparatus.